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Pulsing LEDs to achieve maximum output?

Discussion in 'Misc Electronics' started by Myron Samila, Apr 16, 2004.

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  1. Myron Samila

    Myron Samila Guest


    I've read some discussion here about pulsing LEDs to achieve higher output without burning
    the stupid things out. (Instead of CW).

    I'm just working on a circuit for general purpose low voltage illumination, but want to
    achieve maximum output.

    I suppose a pulse of 100Hz would be decent? This would allow more voltage to be applied
    to the LED without overheating it.

    So I was thinking of using a 555timer triggering a transistor (of what type?) handling
    approx 25 LEDs (no specific data on the LED yet, I haven't gotten that far ie: current,
    voltage, etc..., I'm researching that now).

    I have a basic schematic that shows a 555 triggering such a circuit, just what transistor
    should work for me if I want to lets say switch a 6VDC/1A load?

    Any info would be great, thanks!

    BTW: I believe it was "Watson" that was telling us about his white LEDs that weren't
    lasting very long, well, I've been seeing a lot more white LED applications such as
    crossing lights, while the duty cycle is quite low, I suppose they expect these crossing
    lights (you know the picture of the person walking!) to last quite a while.

    From what I understand, the Osram LEDs are of excellent quality, and priced a wee bit
    higher than the rest as well.
  2. R.Lewis

    R.Lewis Guest

    Read the last thread and you will see that pulsing leds REDUCES the output
    compared to that obtained by the same average current at dc.
  3. Myron Samila

    Myron Samila Guest

    I guess the theory is you could supply more V at the junction, letting it cool in the off
    period, and get higher output, but you're also switching it on and off (quite possibly
    reducing output than if CW)

    The idea is to get the LEDs to last long, ideally, my simple illumination project would
    consist of nothing more than a simple bank of LEDs, and a voltage regulator! (skipping
    any of the pulsing)
  4. Gordon Youd

    Gordon Youd Guest

    Why not use a few more LEDs then run at a lower setting.

    More light less loss!!

    Regards, Gordon.

  5. CWatters

    CWatters Guest

    The problem is the eye sees the average brightness. Put simply if you double
    the power into the LED but for only half the time you get no change. (Even
    that assumes that the LED power efficiency is constant for all power

    This trick does work under certain conditions but typically the frequency
    and mark space ratio has to be low enough for the flashing to be seen. For
    example 0.25 second ON, 2 seconds OFF.

    Actually you need a current regulator not a voltage regulator but that's
    being picky.

  6. Myron Samila

    Myron Samila Guest

    Hi Colin,

    Curious, lets say I have 10 LEDs that draw 20mA each, so 200mA total at lets say the
    breakover voltage is 2.0V, wouldn't a 7802 (if they even have regulators that small, 7805
    common enough) be able to regulate it?

    With the "7802" regulator, you'd be able to handle 50 LEDs (1A) I suppose, if my
    calculations are correct.

    I just don't want to burn out a ton of LEDs cuz I could have done something different,
    research first, then try it ;). Carpenters measure twice, cut once.
  7. CWatters

    CWatters Guest

    I did say I was being picky....

    LED's act like a "constant voltage sink" so if you want to connect them to a
    voltage source you need to put something in series to limit/regulate the
    current. You can use a resistor in series to limit the current.

    Using your examples...

    R = (Vs-Vled)/I
    R = (5-2)/0.02
    = 150 R.

    and the power dissipated in the resistor is

    P = 3 * 0.02
    = 60mW

    so 1/8th W resistors are OK.

    You could try putting two LED in series with each resistor and recalculate
    the value needed as...

    R = (5-4)/0.02
    = 50 R

    and then

    P = 20mW

    Basic maths says that if you want an accurate answer to an equation (eg
    accurate control of the current) you should avoid subtracting one number
    from another similar one (eg avoid Vs-Vled approaching 0!).

    On the other hand the larger the voltage drop across the resistor the less
    sensitive the curcuit is to resistor and LED tollerance - so you may not
    need a voltage regulator at all (but watch the power dissipation in the
    resistor if it's very large).

    Overall you need to balance these issues.

    What is the source voltage? (eg input to the regulator).

  8. Myron Samila

    Myron Samila Guest

    Essentially, Vs can be just about anything at this point, I will design around the load
    requirements. It will be stepped down from AC, rectified and filtered.

    Thanks for the info, I'm trying to source out a local supplier for some white LEDs of good
    quality and of the output I require (the more mcd the better).
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